This invention relates to the control of fuel injection valves in an internal combustion engine based on engine crank angle and air intake signals.
German Offenlegungsschrift No. 27 09 187 discloses a fuel injection control system in which pulses from an ignition pulse generator are shaped and timed as crank angle position or angle-of-rotation signals and supplied to a multivibrator control unit. Each crank rotation of 180.degree. provides an integrated engine speed-dependent voltage signal to a memory in the control unit. Integration is followed by discharge at a rate that depends on the rate of air intake to the engine. The control unit generates corresponding pulses that activate the fuel injection valves. Thus, the duration of the injection control pulse depends on the instantaneous engine speed and the air flow rate. Downstream from the multivibrator is a multiplier that corrects the duration of the pulse in accordance with signals from additional sensors which detect various operating parameters of the engine.
Such conventional control systems have the disadvantage that the engine speed is detected by analog loading of the memory in the control unit before the activating pulse is actually generated. Consequently, the duration of the activating pulse is based on an engine speed value that may no longer be correct. Thus, conventional injection control systems may produce a pulse that activates a downstream injection valve when the duration of the pulse, which determines how much fuel is to be injected, is partly or completely inappropriate for the engine speed at the time when the pulse is generated and the fuel is injected.
This drawback is particularly severe when an engine idling at low speed is subjected to a load. In those circumstances, conventional systems based on detection of engine speed will yield too high a result, and the duration of the valve-activating pulse will reduce the speed even though the actual speed may have already dropped subsequent to the engine speed detection because, for example, an electric load has been turned on. In the event of a misfire at this time, the engine's crankshaft and flywheel may no longer have enough kinetic energy to produce the compression required for the next cylinder and the engine will stall. Although this situation could, of course, be counteracted by increasing the prescribed idling speed, this would increase fuel consumption and could violate environmental regulations.
Such idling behavior of an internal combustion engine depends on certain conditions. Variations in motor speed during idling are due, for example, to variations in the individual burns that can even extend to misfires and to turning on and off the various electric loads in the motor vehicle containing the engine. Since the flow of combustion air through the opening of the throttle valve and into the intake manifold of the engine is hypercritical, occurring, that is, at the speed of sound, the rate of air flow will remain constant even when the engine speed changes. To attain a stable idling speed, the fuel must also be supplied at a constant rate. Because the fuel consumption of the engine during idling is proportional to engine speed even though the given fuel flow remains more or less constant, an idling speed appropriate to a prescribed fuel flow will be established regardless of whether or not the prescribed flow of air and fuel is uniformly distributed among several cylinders during a given time. Then, when a load is applied, a lower engine speed will become established at which the fuel consumption of the engine at idling plus the additional consumption due to the load will again be related to the output of the engine. The previously mentioned minimum speed at which the engine can idle without stalling occurs because the kinetic energy of the mechanism driving the crank and flywheel is proportional to the square of the speed. At some point as the speed decreases, this value will no longer be high enough to provide the compression required for the next cylinder subsequent to a misfire, for example.
The result of these circumstances is that all conventional methods and devices that rely upon fuel injection activating signals having a duration, i.e. a width measured in time, and controlled in accordance with a previously determined engine speed are unable to assure a stable idling speed at a stoichiometric air ratio.
Thus, assuming a fictional operating point with a stoichiometric proportion of air, if the air flow remains constant when an electric load is turned on or there is a misfire while the engine is idling and the quantity of air per combustion chamber increases while the flow of fuel decreases, the amount of fuel per combustion chamber will remain constant. The result is an increased air-to-fuel ratio. As a result, firing will shift into the expansion phase, the amount of work done per cylinder will decrease, and speed will decrease until the engine stops completely. On the other hand, when the speed increases, the flow of fuel will increase and the ratio of air to fuel will decrease. The speed will continue to increase until the air-to-fuel ratio has dropped to a level where it decreases the force output per cylinder.
With such conventional systems as discussed herein, an attempt has been made to establish a stable idling speed at stoichiometric air ratios by permanent readjustment of the fuel supply in response to variations in engine speed. The engine speed is continuously detected and a corresponding fuel injector-activating pulse duration is obtained from a stored graph of engine operating characteristics. The injection time is then corrected with signals from a lambda probe. The result is to produce necessarily unstable injection time readjustments as the speed decreases and the deviations in speed increase. In such conventional systems, the speed data are not available until after a delay of half a crankshaft rotation and, in the case of other control systems, until after a delay of a whole crankshaft rotation. The signal from the lambda probe is not available until considerably later, when the particular combusted mixture has arrived at the lambda probe in the exhaust line of the engine. This delay of the lambda signal is the major engine control problem in conventional systems. It is the major reason why the attainable minimal idling speed depends essentially on the control method and not on the engine itself. This is also true, by the way, when the air intake is regulated in addition to the fuel intake by controlling the cylinder intake with the engine idling.
In contrast to these conventional control systems, the control system described in German Patent No. 32 19 007 remotely detects not only the beginning but also the width of the pulses that activate the fuel injection valves. This is done with sensors, Hall generators, for example, mounted on a disk that rotates dependently of engine speed and that has two pulse generators mounted on it at prescribed angular intervals. These controls, however, operate mechanically and are not able to carry out regulation in accordance with such other parameters as temperature or signals from a lambda probe. Furthermore, the accelerator position does not provide unambiguous information about air flow in all of the operating conditions of the engine.